(1) OFDM System and Transmission Format (Pilot Signal)
OFDM has been applied to a variety of types of communications systems. As an example of OFDM, the following non-patent document 1 examines a cellar system employing OFDM. The system of this non-patent document 1 describes that a pilot signal [called “reference symbol” (RS) in the non-patent document 1] having been already known in transceiving used in channel estimation or the like is sent at the two-dimensional allocation of the time and the frequency as illustrated in, for example, FIG. 11 (see section 7.1.1.2.2 in non-patent document 1).
That is, according to the example illustrated in FIG. 11, in a single sub-frame (=0.5 ms=7 symbol time periods), the first pilot signals (1st reference symbols) R1 are allocated (mapped) to the leading symbol of the sub-frame at a seven sub-carrier cycle, and the second pilot signals (2nd reference symbol) R2 are allocated to the fifth symbol of the sub-frame at a seven sub-carrier cycle (here, allocated to sub-carriers different from the first pilot signal R1). In this instance, a variety of types of data (D) are allocated to the other symbols.
(2) Estimation of Interference Noise Power
A formula for estimating interference noise power is indicated as the formula (13) in the following non-patent document 2 (see p. 1560). This method uses a channel estimation value at the position of each pilot signal in the frequency domain at the same symbol time (the same reception time). The channel estimation value A at a certain position and the average value B of the channel estimation values of the neighboring pilot signals in the frequency direction are calculated, and interference noise power is estimated based on the average power of a difference there between (A-B).
(3) Use of Interference Noise Estimation Value
Here, the interference noise estimation value is used on a receiver for various purposes.
For example, the following non-patent document 2 describes an example of reception processing by use of the interference noise estimation value. The interference noise estimation value is used for a part of MIMO demodulation processing.
Further, paragraph [0058] in the following patent document 1 describes that a fading coefficient/noise power is publicly known as the maximum ratio combination coefficient of more than one branch, and also describes that the noise power estimation value is used in reception processing.
In this manner, since the interference noise power is used in a variety of types of communications systems and receivers, to estimate interference noise power with high accuracy is significantly important. If correct estimation is unavailable, it leads to deterioration of the reception (demodulation processing) ability of a receiver.
Patent Document 1: Japanese Patent Application Publication (Laid-open) No. HEI 7-202758
Non-patent Document 1: 3GPP TR25.814 V1.5.0 (2006.5)
Non-patent Document 2: H. Kawai, et al., “Independent Adaptive Control of Surviving Symbol Replica Candidates at Each Stage Based on Minimum Branch Metric in QRM-MLD for OFCDM MIMO multiplexing”, IEEE VTC2004-Fall. Vol. 3, pp 1558-1564